The present invention relates to marine anchors and particularly to drag embedment and direct embedment anchors and their embedment means.
A marine anchor for embedment in a mooring bed is attached generally to an anchor line for connection to an object to be restrained by mooring in a body of water over the mooring bed. The anchor includes a load application point for the attachment of the anchor line thereto via anchor line attachment means (for example, a shackle) and a fluke member and includes a plane of symmetry containing a first direction in which the surface of the fluke member viewable from the load application point when the anchor is in operation has a maximum projected area and a second (forward) direction (F) in which said surface has a minimum projected area. Correspondingly, in these directions maximum and substantially minimum resistance to movement of the anchor in a mooring bed soil occurs. The anchor fluke tends to advance in the soil along the forward direction (F) of minimum resistance.
A drag embedment anchor is a marine anchor as described above wherein the anchor line attachment means load application point is located on the anchor such that pulling horizontally on the line with the anchor lying on the surface of a mooring bed causes the anchor to tilt into penetrative engagement therewith and then moves into the mooring bed soil with a substantial component of displacement occurring in the forward direction of minimum projected area of the fluke member surface. This causes the anchor to follow a curved burial trajectory as it embeds into the mooring bed soil. The location of the load application point thus allows the anchor line attachment means to function as the embedment means of the anchor.
A direct embedment anchor for example EP-A-0161190 is a marine anchor as described above which has an anchor line attachment means load application point located such that pulling on the attached anchor line causes the anchor to tend to move in the direction of maximum projected area of the fluke member when buried in the mooring bed soil. This causes the embedded anchor to follow a path that rises to and breaks out through the mooring bed surface and so prevents the anchor line and anchor line attachment means from functioning as the embedment means of the anchor. An alternative embedment means is therefore employed which comprises a pushing member, known as a follower, to engage with and push the anchor deep into the mooring bed soil substantially in the forward direction of minimum projected area of the fluke member.
Each anchor before-mentioned will hereinafter be referred to respectively as a marine anchor, a drag embedment anchor or a direct embedment anchor of the type described hereinbefore.
These anchors have disadvantages: the drag embedment anchor requires a sometimes unacceptable horizontal component of displacement to reach a desired embedment depth below the surface of a mooring bed and the direct embedment anchor suffers from a progressively reducing embedment depth when overloaded which ultimately results in catastrophic failure by breaking out of the mooring bed. Further, the direct embedment anchor requires to be pushed into the seabed by a long follower that is prone to being damaged and is difficult to handle when decking on an anchor-handling vessel.
The objectives of the present invention include inter alia mitigating these disadvantages. The present invention broadly provides anchoring apparatus comprising a marine anchor that follows a burial trajectory when dragged by an anchor line via an anchor line attachment means after being embedded to an initial buried position below a seabed surface and embedment means for establishing the initial buried position.
According to a first aspect of the present invention, a marine anchor as hereinbefore described and in operational configuration for operation below the surface of a mooring bed is a drag anchor characterised in that a straight line containing the load application point and the centroid of the fluke member surface viewable from the load application point forms a forward-opening angle (xcex2) with the forward direction (F) in the range 68xc2x0 to 85xc2x0 for operation in soft cohesive soil and in the range 50xc2x0 to 65xc2x0 for operation in non-cohesive soil whereby a pulling force applied to the anchor by the anchor line at the anchor line attachment means load application point when the anchor fluke centroid is buried at least twice the square root of said maximum projected area below the mooring bed surface causes the anchor to tend to move in the soil of the mooring bed with a substantial component of displacement in the second forward direction.
Preferably said substantial component of displacement in said second forward direction exceeds 35 per cent of the actual displacement.
Further preferably said substantial component of displacement in said second forward direction exceeds 50 per cent of the actual displacement.
Preferably said centroid angle does not exceed 80xc2x0 for operation in soft cohesive soil and does not exceed 60xc2x0 for operation of non-cohesive soil.
Preferably said drag anchor is further characterised in that a plane orthogonal to the plane of symmetry of the anchor and containing a forward extremity of the fluke member and the loan application point forms a forward-opening angle (a) with the forward direction (F) which is not less than 95xc2x0 for operation in soft cohesive soil and not less than 85xc2x0 for operation in non-cohesive soil.
Preferably said point angle is not less than 100xc2x0 for operation in soft cohesive soil and is not less than 90xc2x0 for operation in non-cohesive soil.
Preferably the drag anchor according to the first aspect of the present invention comprises a fluke with a plate-like shank member rigidly attached thereto and lying parallel to said plane of symmetry.
Preferably said plate-like shank member includes an elongated slot for slidable movement therein of an anchor line attachment means, with a forward end of said slot serving as an anchor line attachment means load application point permitting deeper burial of the anchor by dragging and with a rear end located towards a rear edge of said fluke serving as a substitute anchor line attachment means load application point permitting easy rearwards recovery of the anchor in a direction substantially opposite to said forward direction.
Preferably a slide stop means is provided just aft of the forward end of said slot to restrain said attachment means at said load application point.
Preferably said slide stop means includes release means which cooperate with said anchor line attachment means whereby rotational displacement of said attachment means releases said slide stop means to permit said attachment means to slide in said slot towards the rear of said fluke.
Preferably said anchor line attachment means comprises an elongate shackle.
Further preferably said anchor line attachment means comprises an elongate member with an attachment point at one end serving for connection to an anchor line and with a clevis at another end carrying a pin member serving to engage slidably and rotatably in said slot in said shank member.
Preferably said shank member includes an arcuate surface centred on said load application point and said elongate member includes a stop slidably engageable on the arcuate surface whereby said pin member is held at the load application point in said slot until rotation of the elongate member about the load application point brings the direction of movement of the stop parallel to the slot whereupon the pin member is free to slide in the slot.
Preferably said anchor includes releasable rotation stop means which stops rotation of said elongate member at a predetermined position relative to said shank member when said pin member is at said load application point.
Preferably the length of said elongate member is such that, when the member is stopped from rotating by said releasable rotation stop means, a plane lying orthonogal to said plane of symmetry and containing a forward extremity of said fluke member and said attachment point on the elongate member forms a forward-opening angle with said second direction which does not exceed 95xc2x0 and further preferably does not exceed 75xc2x0.
According to a second aspect of the present invention, a marine anchor and embedment means comprises one of a drag embedment anchor as hereinbefore described and said drag anchor, and an elongate follower member detachably attached thereto and adapted for pushing said anchor, substantially in said second forward direction of minimum projected area of the surface of said fluke member viewable from said anchor line attachment means load application point, until the anchor fluke centroid is at least twice the square root of said maximum projected area below the surface of a mooring bed whereby subsequent pulling on the anchor line after detachment of the follower member from the embedded anchor causes the anchor to tend to move in the soil of the mooring bed with a substantial component of displacement in said second direction.
According to a third aspect of the present invention, a marine anchor and embedment means comprises one of a drag embedment anchor and a direct embedment anchor and a drag anchor as hereinbefore described and an elongate follower member detachably attached thereto and adapted for pushing said anchor substantially in said second direction into a mooring bed characterised in that at least one of said anchor and said elongate follower is adapted to provide a reaction fulcrum about which the anchor may pivot.
Preferably said marine anchor is adapted for pivoting about said fulcrum when a pulling force is applied to the anchor by an attached anchor line.
Preferably said embedment means for directly embedding a marine anchor comprises an elongate follower member adapted to provide detachable attachment to a marine anchor and a reaction fulcrum about which the anchor may pivot when pushed into a mooring bed by said follower member.
According to a fourth aspect of the present invention, a marine anchor and embedment means comprises a marine anchor as hereinbefore described and an elongate follower member detachably attached thereto and adapted for pushing said anchor substantially in said second direction and further adapted to bend recoverably without suffering damage when subjected to transverse forces, for example, due to traversing a curved surface such as a stern roller of an anchor handling vessel.
According to a fifth aspect of the present invention, an embedment means for directly embedding a marine anchor comprises an elongate follower member adapted for detachable attachment to a marine anchor and further adapted to bend recoverably without suffering damage when subjected to transverse forces, for example, due to traversing a curved surface such as a stern roller of an anchor handling vessel.
Preferably said follower member includes a lower terminal segment attached to a lowering and recovering line and inlcudes a plurality of body segments supported by said lower terminal segment.
Preferably said body segments substantially encircle said lowering and recovering line.
Preferably said segments fit together by means of a convex protuberance on a segment registering with a corresponding concave recess on an adjacent segment.
Preferably said lowering and recovering line forms an axis passing through said body segments.
Preferably at least a portion of said line within said body segments comprises at least one of a rope and a chain.
Preferably at least a portion of said line within said body segments is formed of resiliently extensible material such as, for example, polyester rope.
Preferably when said line within said body segments is extended under tension when said follower is hanging vertically, said line is prevented from relaxing by a line stop means acting between an upper body segment and said line whereby said body segments are maintained in a state of axial compression which provides said elongate follower member with a degree of transverse stiffness to resist buckling when said follower is at least partly supported by contact with a sea bed surface.
Preferably said line stop means on said upper body segment is releasable whereby, when said follower is pulled up and bent over said curved surface, said line is released within the follower to allow relative axial movement between the line and the upper body segment to avoid excessive stretching of the line due to bending of the follower.
Preferably said line stop means is releaseable by means of movement of an actuator making contact with said curved surface.
Preferably said line stop means includes a tooth member located on one of said line and said upper body segment which engages in a recess in a recess member located on the other one of the line and the upper body segment.
According to a sixth aspect of the present invention, an embedment means for embedding said drag anchor comprises an anchor line attached thereto via an elongate rigid member anchor line attachment means, said elongate member having a first attachment point at one end serving for attachment to the anchor line and a second attachment point at another end for attachment to said anchor line attachment means load application point on the anchor, and releasable rotation stop means for holding the elongate member relative to the anchor such that a plane orthogonal to said plane of symmetry containing a forward extremity of said fluke member and said first attachment point forms a forward-opening angle with said second direction which does not exceed 750 to promote penetration of a mooring bed surface when the anchor is dragged thereover but which releases due to soil loading on said fluke as said fluke becomes buried in the mooring bed soil.
Preferably said elongate rigid member has a clevis at said second attachment point which carries a pin member serving to engage slidably and rotatably in said slot in said shank member of said drag anchor.